or afghanistan, there are signatures of those types. >> and fbi investigators as it will take some time to examine the evidence. authorities are using security camera tape and cell phone video that returned in to police. >> d.c. and other cities have increased security at landmarks, and buildings and transit after this. pennsylvania avenue outside the white house was closed to pedestrian traffic and metro transit police have canine units and extra patrols at l'enfant plaza and other stations. there's no specific threat against d.c., but city leaders are not taking any chances. >> any time there's an incident anywhere nationally or internationally, until we have had enough information to feel comfortable there is no potential connection to washington. >> the university of maryland, maryland state police, and restored as county police have also increased security. an emotional tribute to victims of the boston explosion. >> some students at george washington university held a vigil just hours after the bombing. one student who was a boston native says that he's devastated by what happened.